The overall performance of a complex digital electronic circuit, such as a microprocessor, is often limited by the speed of the slowest electrical path within that circuit. Often, this path involves a capacitively loaded line. Each driver or receiver circuit on a line adds some capacitive load to a line. In addition, there are coupling capacitances to structures in adjacent layers and to adjacent lines within the same interconnect layer. In modern IC processes, line spacing is decreasing. Adjacent-line coupling capacitances within the same interconnect layer are becoming higher than the coupling capacitances between layers and are becoming higher than coupling capacitances for previous generations of IC processes. In submicron IC processes, capacitance to adjacent lines on the same interconnect layer may be the largest single contributor to overall line capacitance. One result is that lines are becoming more susceptible to noise induced by the transition of one or more adjacent lines on the same interconnect layer. If noise is great enough it may result in erroneous operation.
There are two common solutions to driving heavily loaded lines. The first solution is to use push-pull drivers in which the driver actively drives the line either high or low. Both the pull-up and pull-down devices must provide sufficient current to drive a capacitively loaded line to a logical one or zero within a specified fraction of a clock cycle. Additionally, both the pull-up and pull-down devices must provide sufficient drive strength (current) to suppress noise coupled onto the line. However, the ability to rapidly drive the line both low and high comes at the expense of size and line capacitance as discussed below.
In a typical push-pull driver designed in complementary-metal-oxide-semiconductor (CMOS) technology, a p-channel device is used for pull-up and an n-channel device is used for pull down. For an MOS transistor, the saturation value of the drain current I.sub.DO is given by the following equation: ##EQU1##
In the above equation, .mu..sub.S is surface mobility of the majority carriers, C.sub.OX is the capacitance per unit area of the gate electrode, W is the width of the transistor, L is the length of the transistor, V.sub.GS is the voltage from gate to source, and V.sub.TH is the threshold voltage. Note that .mu..sub.S for holes (p-channel devices) is in the range of 150-250 cm.sup.2 /V-sec and .mu..sub.S for electrons (n-channel devices) is in the range of 300-600 cm.sup.2 /V-sec.
From the above equation, in a push/pull driver, to provide the same rise time as fall time, the size (W/L) of the p-channel pull-up transistor must be about 2.4 times the size (W/L) of the n-channel pull-down transistor. This leads to increased area requirements for the overall integrated circuit to accommodate the large p-channel pull-up transistors. In addition, capacitive loading imposed on a line by a transistor is directly proportional to the size of the transistor, so the large p-channel pull-up transistors increase the capacitance on each driven line. Therefore, it is desirable to eliminate the need for large pull-up transistors.
In a second common solution for driving heavily loaded lines, the line is precharged through a single precharge transistor during a precharge phase of a clock. Then, during a drive phase of the clock, pull-down drivers optionally discharge the line. All the driver pull-up transistors are eliminated (replaced by a single precharge transistor), reducing the circuit area and the capacitance on the line. In addition, the solution takes advantage of the smaller size of the n-channel pull-down transistors. However, during the drive phase, lines that are not discharged are left at a high impedance (floating), with potential susceptibility to noise. In particular, in submicron IC processes, precharge/pull-down bus structures are prone to failure due to the increased cross-coupling to adjacent lines.
In general, push-pull drivers provide lower susceptibility to coupling noise at the expense of increased area and increased line capacitance, whereas precharged lines provide smaller area and lower line capacitance at the expense of increased susceptibility to coupled noise. There is a need for low noise lines as provided by active push-pull drivers but with a smaller circuit area.